[Cashtab] OP_RETURN prefix protocols

Summary: Markdown file and CSV added to provide context and information regarding XED OP_RETURN prefix protocols

Test Plan:
cd web/cashtab
inspect xec-protocols.csv and op_return-prefix-guidelines.md
ensure information is correct and advise on what is missing.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10702